1. Field of the Invention
The present invention relates to data transmission systems and, more specifically, to a data transmission system for transmitting data from a server to a data circuit terminating equipment that is connected to a data terminal equipment. Here, the data that is transmitted from the server is the data that is reserved by the data terminal equipment.
2. Description of the Background Art
There have been proposed various data transmission systems which are of a type as described above. An example is the data transmission system disclosed in Japanese Patent Laid-Open Publication No. 8-140081 (96-140081), which includes a device in an information source (hereinafter, referred to as server), and an information storage unit which receives the information on the user's side. The server and the information storage unit are connected to each other over a network. The server waits for users' requests for data transmission until a data distribution time which has been set under a predetermined manner. Even if one request comes, the server waits for other requests for the transmission of the same data for the set time period. When the data distribution time comes, the server sends the requested data onto a communications circuit. Then, the information storage unit on the user's side receives and stores the data therein. Accordingly, in such a conventional data transmission system, the server can selectively perform data transmission in a time period when, before the data distribution time, the communications circuit is not congested. In this manner, the communications circuit can be efficiently utilized.
The issue here is that the communications circuit varies in characteristics depending on its type. As an example, a wired public circuit typified by ISDN is not suited for multicasting. This is because any transmission bandwidth of a number of data channels in the public circuit is occupied by multicasting, thereby simultaneously transmitting the same data to many users using the public communications circuit. Unlike the wired public circuit, a satellite circuit may be a possibility for multicasting since data transmission is performed through the shared use of a transmission bandwidth. However, the conventional data transmission system still bears a problem of not efficiently utilizing the transmission bandwidth of the communications circuit since data is unconditionally sent out to the same communications circuit regardless of the number of users receiving the data. Here, assume that the data transmission system only has a wired public circuit. In such a case, a server in the system has to transmit data through the wired public circuit regardless of whether multicasting is preferable, which results in a waste of transmission bandwidth.
The conventional data transmission system bears another problem regarding a communications expense. In a case where a user wants to download relatively large data such as moving pictures, the communications expense therefor is high.